Conditions such as asthma can be managed by measuring the concentration of nitric oxide (NO) in the air exhaled by a subject. Those suffering from asthma produce elevated levels of NO in the lower respiratory tract (the bronchi and alveolar part).
Currently, for a NO measurement to be performed, the patient must exhale at a constant air-flow rate of 50 ml/s for 10 seconds. Due to a flow restrictor, the pressure in the mouth cavity of the subject is high enough to close the velum—the soft palate consisting of muscle fibers sheathed in mucous membrane responsible for closing off the nasal passages during the act of swallowing and sneezing—thereby preventing NO from the nose from disturbing the measurement.
However, in this method, NO concentrations are extremely low, and a sensitive and expensive system is required to measure the parts-per-billion concentrations of NO in exhaled air and to integrate the measurements over time. Even so, the result is prone to errors and offset due to NO produced by glands in the upper airways of the subject.
Furthermore, the current detection techniques are based on the subject maintaining a near constant exhalation for a period of 10 seconds, which is not easy or even possible for older subjects, young children or subjects having difficulty breathing. The relatively large volume of exhaled gas is required in order to achieve a sufficient signal-to-noise ratio (SNR) in the NO measurement. Furthermore this method is not ideally applicable for use in continuous monitoring of subjects.
Furthermore, in the case where the NO concentration is measured during normal breathing (i.e. in continuous monitoring) the nose has to be blocked to avoid errors and offset due to residual NO production in the nasal glands. This blocking is very inconvenient for the subject.
Therefore, there is a need for an improved method and apparatus for measuring the concentration of nitric oxide (and other specific gases) in an exhaled gaseous stream.